Electron multiplier and method of manufacturing dynodes



March 7, 1967 ELECTRON MULTIPLIER AND METHOD OF MANUFACTURING DYNODES R. L. VAN ASSELT Filed Maron so,V 1962 INVENToR. ROBERT L.\/AN AssELT ATTORNEY United States atent fiee 3,308,324 Patented Mar. 7, 1967 3,308,324 ELECTRON MULTEPLIER AND METHOD GF MANUFACTURlNG DYNODES Robert L. Van Asseit, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Filed Mar. 30, 1962, Ser. No. 183,802 3 Claims. (Cl. 313-65) This invenion relates to electron multipliers. In particular this invention relates to an improved electron multiplier for use in a photosensitive type of pickup tube.

One of the problems encountered in the selection of a material for use as an electron multiplier structure which is to be used in a photosensitive pickup tube, is that of selecting a material that provides a uniform secondary electron emission from all emitting areas of the multiplier. When the secondary electron emission is not uniform from all emitting areas, the signals resulting from the emission tend to provide a shading or non-uniformity of the original signal.

Another factor in the selection of materials for use as an electron multiplier is that secondary electron emission should be constant during the normal tube life. When the secondary electron emission is not constant, the signal from the multiplier will vary with the age of the device, which normally results in a relatively ineiicient electron multiplier after the device has been used for a particular length of time.

The problems of obtaining a uniform electron emission and maintaining this electron emission constant from a secondary electron multiplier are particularly pronounced in certain photosensitive pickup tubes. One type of photosensitive pickup tube in which these problems are of particular importance is that used in television and is known as an image orthicon. If a non-uniformity of the secondary electron emission exists in the first dynode of the electron multiplier of an image orthicon, the output signal from the tube is distorted by the superposition of spurious signals, introduced by the non-uniformity in secondary emission. The spurious signals are known as bad shading by those skilled in the art.

The causes of bad shading in a pickup tube are difficult to determine. However, it has been found that one of the more important factors producing bad shading is that of dynode burn. Dynode burn is a change in the secondary electron emission of limited areas of the iirst dynode surface during the operation of the tube. The cause of this change is secondary electron emission is not clearly understood. However, it has been found that the dynode burn is caused partially by the simultaneous `action of electron bombardment on the iirst dynode, in conjunction with high ambient temperatures such as those caused by heat radiated from the cathode of a closely adjacent electron gun.

Dynode burn problems are amplified in television pickup tubes due to the fact that, during the formation of many of the coventional photocathodes used in pickup tubes, first oxygen and then cesium is introduced into the tube. Therefore, there is a possibility that areas of the surface of the dynode will become oxidized, and this oxide binds to and/ or absorbs by adhesion, some of the cesium that is present in the tube. Under the joint influence of heat and electron bombardment, during the subsequent tube operation, some of the cesium may be removed from the surface thus causing a decrease in the secondary electron emission from an area of the dynode.

The problems of dynode burn have been solved in the prior art. For example, in U.S. Patent 2,942,132 to Rotow et al., issued lune 21, 1960, there is provided a dynode including -chromium which has the extremely desirable property of being substantially free of dynode burn. However, due to the relatively low secondary electron emission from chromium, the chromium dynode surface provides little or no gain. In other words, the chromium dynode is used for its property of freedom from dynode burn at a sacrifice of the signal gain, or electron multiplication, from the first dynode.

It is therefore an object of this invention to provide a new and improved dynode for an electron multiplier characterized by its high, uniform, and constant secondary electron emission during tube operation.

It is another object of this invention to provide an improved iirst dynode for a secondary electron multiplier characterized by its stability under conditions of tube processing, manufacturing and operation and by its high gain and high signal to noise ratio during tube operation.

It is another object of this invention to provide an improved pickup tube characterized by its freedom from dynode burn coupled with a high output signal level for a given input light level.

These and other objects are accomplished in accordance with this invention by making the irst dynode of a material that is stable with respect to the photosensitive materials used in the tube, and of a material that has a high, constant, secondary electron emission. The material which has been found to meet these requirements is magnesium oxide applied over a base layer prepared in a novel way.

This invention will be more clearly understood by reference to the accompanying single sheet of drawings Wherem:

FIG. 1 is a longitudinal cross section of a pickup tube of the type incorporating this invention; and,

FIG. 2 is an enlarged sectional view of the iirst dynode structure of the tube of FIG. 1.

Although the invention is applicable to all types of tubes wherein an electron multiplier arrangement is used, it is particularly applicable to photosensitive pickup tubes using an electron multiplier structure. Furthermore, for simplicity of illustration, the invention will be described with particular reference to a pickup tube of the image orthicon type as an example.

Referring to FIG. 1, there is shown a sectional view of an image orthicon type pickup tube 10. The tube comprises an electron gun 16 in one end of an evacuated envelope 11. The electron gun 16 is a well-known type and further description thereof is not deemed necessary. The electron gun 16 is for the purpose of producing an electron beam 14 which is accelerated, by conventional accelerating electrodes, toward a target electrode 18 with potentials such as those shown in FIG. 1. The target electrode 18 is a semiconductor, e.g. glass or magnesium oxide, and is supported transverse to the electron beam 14. Mounted immediately adjacent to the target 18 is a decelerating electrode mesh screen 20 which brings the velocity of the electron beam 14 to substantially zero in front of the target surface. The electron beam 14 will land lon the target and drive the front or scanned surface of the target 18 substantially to cathode potential after which the electron beam is reected by the target as a return electron beam 15. The fields, produced by a deflection yoke 19, and a focus coil 21, which cause the electron beam to scan the target, also cause the return beam 15 to scan a iirst dynode electrode 24.

At the end of the tube 10 opposite the gun 16, and on the side of target 18 remote from the gun 16, there is formed a photocathode 22 which emits electrons in proportion to the amount of light focused thereon from -a scene to be reproduced. The photocathode 22 normally includes an oxidized and cesiated silver alloy or a film of antimony activated with small amounts of a plurality of alkali metals. Examples of conventional photocathodes may be found in U.S. Patent 2,682,479 to Johnson, and U.S. Patent 2,770,561 to Sommer.

The electrons from the photocathode 22 are accelerated and focused onto the back of the target 18, during tube operation, to produce, by secondary electron emission, a charge pattern which appears on the side of the target 1S toward the electron gun 16. When the electr-on beam 14 is scanned over the front surface of the target 18, the beam is reflected from the areas of the target 18 that are not charged by the photocathode 22 since these areas are substantially at cathode potential. The beam 14 lands on the areas of the target 18 that are charged by the photocathode until the Acharge pattern is neutralized by the electron beam 14. Once the charge is removed, the balance of the beam is reected toward the electron gun 16. Thus, the return electron beam is modulated in proportion to the amount of charge on the target 1S.

The electron beam 15 returns toward the electron gun 16 and scans over a rst dynode 24 that surrounds and forms an aperture 1'7 in the rst accelerating electrode for the primary electron beam 14. The return electron beam 15 is multiplied by secondary electron emission of the rst dynode 24 and is directed into an electron multiplier 26. Generally, the electrons from the rst dynode 24 will land with a scattered distribution over the area of the second dynode so that areas of the second dynode that are of a non-uniform secondary emission are not a pronounced problem. The electron multiplier 26 may be of any type such as that disclosed in U.S. Patent 2,433,941 to P. K. Weimer. The modulated return electron beam is converted into output video signal voltages from a collector electrode of the electron multiplier 26. i During the manufacturing process of tube 10, certain reactive materials, such as, for example, antimony, cesium and oxygen, are introduced within the tube to form the photocathode 22. Some of these materials may land on the first dynode 24 either during manufacturing or subsequently during tube operation since the materials may be occluded in the electrodes or the envelope walls during tube processing and be gradually released during tube operation.

The proximity of the rst dynode electrode 24 to the electron gun 16 should be noted. Due to this proximity, the first dynode electrode 24 is heated by heat radiated from the heater of the electron gun 16. Due to the presence of the reactive materials and the heat from the cathode, certain reactions have occurred in the prior art tubes which have resulted in dynode burn or change in the secondary emission of the first dynode surface. As was explained, dynode burn produces spurious signals in the output signal as the return beam 15 scans over the non-uniform secondary emitting areas of the first dynode.

The first dynode 24, which is shown more clearly in FIG. 2, is made of a material which has a high, constant and uniform secondary electron emission. It has been found that a material which meets these requirements is magnesium oxide. It has also been found that the magnesium oxide material should be generated on a Very clean substrate which has the property of being free of dynode burn. Examples of such materials are chromium, nickel and gold. For example, the irst dynode electrode 24 may be made of solid chromium with an exposed surface layer of magnesium oxide thereon. In the alternative, the magnesium oxide may be deposited on a chromium layer 28 which has been coated onto a base metal 30. The base metal 30 may be a material such as silver, nichrome or nickel. In either case, a layer of chromium, nickel or gold should be applied while the rst dynode is in the evacuated chamber in which the magnesium metal is to be applied. The coated base structure is preferred due to the fact that a small aperture 17, eg. 2 to 4 mils, for the electron beam must be drilled in the rst dynode, and the base metals suggested are' easier to drill than solid chromium for example.

Assumingthat a base metal is used, the deposit of the chromium, nickel or gold layer 28 is performed in a standard, high vacuum, oil pumped vacuum system (not shown) with a liquid nitrogen cold trap. The method of deposition may be by evaporation, such as by bombarding an evaporator boat, containing the selected metal, with an electron beam and depositing the material evaporated thereby onto the substrate Sil. Although the thickness 4of layer 28 is not critical, it is critical that the substrate be clean. Therefore, even though the base metal 30 may be chromium, additional chromium should be evaporated and deposited as the layer 28, after the dynode 24 is in the vacuum system in which the magnesium metal is to be deposited, and the magnesium deposit made without otherwise breaking the vacuum, to insure the presence of a clean substrate.

The provisions of a clean substrate may be optically monitored by placing a clean glass plate in a jig (not shown) used to support the dynodes. The light transmission through the glass plate is then monitored, e.g. by light from a tungsten source (not shown) operating at 2870 color temperature and a phototube (not shown) which measures the light transmitted through the glass as is well known. The transmission is monitored, and the chromium, for example, evaporation continued until a transmission level of approximately 2 to l0 percent, of the original light transmission, is reached. This transmission has been found to provide a clean chromium substrate and is believed to result in a deposit of chromium of approximately l0() to 200 Angstrom units in thickness.

It has been found that a double evaporation of chromium, With the dynode rotated 180 between evaporations, or using two evaporator boats with boats spaced 180 is desirable in certain instances to provide a clean surface. Generally, the two evaporations will be needed when the dynode is particularly rough. In other words, chromium, nickel or gold should be evaporated to substantially isolate the base metal 30, along with any impurities, from the subsequently deposited magnesium ayer.

When chromium metal is used as the layer 28, the chromium should not be deposited to a thickness which will result in cracking. This thickness occurs when the transmission is about 1/2 of one percent. Two to 10 percent transmission will produce the desired result of a clean substrate. If gold or nickel is used as the layer 28, the problem of cracking does not occur.

After a clean substrate or layer 28 is provided, the transmission indicator is reset to and magnesium metal is evaporated, without breaking the vacuum. The evaporated magnesium is condensed onto the clean substrate 28 until the transmission through the monitoring glass is within the range of 40 percent to 80 percent of the reset reading. These light transmission readings result in a deposit of magnesium metal that is believed to be with- 1n the range of approximately 125 A. to approximately 25 A. magnesium thickness.

Oxidation of the magnesium deposit may be accomplished before tube processing e.g. by admitting dry oxygen to the vacuum system at atmosphere pressure and at a temperature of about 450 C. for about 2-3 minutes. Or, the oxidation may be done during tube processing during the standard exhaust process. The oxidation process produces a layer of magnesium oxide 32 which is of substantially the same thickness as the deposited magnesium metal.

The commercial image orthicon normally operates with a dynode voltage of about 300 Volts. The calculated maximum secondary electron gain from a magnesium oxide surface is about 25 and occurs when a voltage of about 1200 volts is used. To obtain this maximum calculated gain a magnesium deposit of approximately 250 A. thickness is necessary. Thus, the optimum gain obtainable is a gain of about 25 for a 250 A. deposit of magnesium oxide and a dynode Voltage of about 1200 volts.

Since conventional image orthicons are designed to operate with a dynode voltage of only 300 volts, the optimum magnesium thickness for this voltage .should be selected. It has been estimated that a gain of about 10 is the maximum that is theoretically possible to attain at this voltage. This maximum gain of is possible only when the magnesium oxide surface is as clean as possible and as free from other materials as possible. In normal image orthicon production, and operation, other materials eg. cesium from the photocathode, will tend to Vary the maximum theoretical gain to a maximum practical gain of about 7 at the conventional 300 volts.

Thus, the thickness of the magnesium is a controlling factor in the gain obtainable. The thicker layer produce higher gains up to a limit depending on the energy of the primary electrons (300 V. in a conventional operation) impinging on the surface. Any increase in thickness of magnesium oxide beyond the optimum results in a reduction of gain because of an increase in the resistance through the magnesium oxide.

In View of the considerations set forth above, for conventional 300 v. operation, a magnesium deposit should be employed which is formed by depositing magnesium until a 40 percent to 80 percent light transmission is obtained compared to the transmission just prior to making the deposit. This is believed to form a deposit of magnesium of 125 A. to 25 A. thick. Within this range, it is believed that a deposit of approximately 60 percent transmission, or approximately 75 A. is optimum for conventional 300 volt operation.

Still further, the voltage drop across a resistive layer may initiate electrochemical reactions in the dynode surface, resulting in dynode burn. Therefore, the minimum thickness required to achieve the desired gain is recommended. The optimum thickness of the magnesium deposit, as a function of voltage on the dynode is almost linear in the region between 0 volts and 800 volts. For example at 450 Volts a 45% transmission (approximately 110 A.) and at 600 volts a 30% transmission (approximately 150 A.) is believed to be optimum. With a voltage of about 500 volts, a magnesium deposit of about 100 A. to 150 A. would result in a gain of 12 to 13 and is optimum for operation at this voltage.

After the desired thickness of magnesium metal is deposited it is oxidized, as has previously been explained, to form the magnesium oxide layer 32. When this has been done, it has been found that a hydrogen tiring of the dynode 24 is `desirable to eliminate any contaminants which may have come from the base material 30, or from the dynode processing. The contaminants, if present, would tend to attract cesium, from the photocathode 22, and would thus produce a non-uniform area of secondary electron emission. The hydrogen firing may be done in a conventional furnace at approximately 725 C., with line hydrogen atmosphere, for approximately 3 minutes.

It has been found that when a dynode electrode is formed of chromium substrate having a magnesium oxide layer 25 A. to 125 A. thick, assuming 300 volt operation, the problems of dynode burn are substantially eliminated in that the chromium underlayer and the magnesium oxide are substantially inert with respect to the materials used in the manufacture `of the photocathode 22. Thus, the combined effects of electron bombardment by the return beam 15, heat from the cathode of gun 16 and stray materials from the photocathode 22 do not produce dynode burn of the lirst dynode 24 when the dynode is made as described. Furthermore, the secondary electron emission of the first dynode 23 under such conditions is high (approaching about 10) and substantially constant with the age of the tube.

Although the invention has been described as used in a television pickup tube, it should be understood that this invention is also applicable to other phototubes of the type including an electron multiplier. An example of a phototube wherein the problem of bad shading is pronounced is the type of phototube that is used in conjunction with -a flying spot scanner for film pickup. In a tube of this type, bad shading produces spurious signals in the output current similar to those previously described.

'What is claimed is:v

ll. In a photosensitive pickup tube comprising a liat target adapted to operate at a relatively low voltage with respect to ground, a heat radiating electron gun for directing electrons to one face of said target, a portion of said electrons returning towards said gun, an electron multiplier having a first dynode adjacent to said electron gun and in the path of said returning electrons, said multiplier being adapted to operate at a relatively high voltage with respect to ground, whereby said returning electrons directly impinge on a surface of said first dynode with appreciable velocity and are deflected to an adjacent dynode, the improvement characterized in that:

(a) said first dynode comprises a composite structure consisting of a base metal selected from the group consisting of silver, indium and nickel, a layer of chromium on said base metal having a thickness of from about Angstrom units to about 200 Angstrom units, and a layer of magnesium oxide on said layer of chromium, said magnesium oxide layer being directly exposed to said returning electrons,

(b) .said magnesium oxide layer having a thickness related to the voltage difference between said target and said multiplier within the range of from 300 volts to 600 volts, said thickness of said magnesium oxide layer being about 75 Angstrom units when said voltage difference is about 300 volts and increasing substantially linearly with increase in voltage to a value of about Angstrom units when said voltage difference is about 600 volts,

(c) whereby said first dynode is characterized by optimum gain and is free from adverse effects from the heat of said electron gun.

2. The method of manufacturing a dynode electrode for use in an electron tube having a photocathode including cesium, comprising the steps of depositing in a vacuum a layer of material on a substrate by evaporation, depositing magnesium only, on said layer by evaporation while maintaining said vacuum, oxidizing said magnesium, and hydrogen firing said dynode for eliminating cesium attracting contaminants from said dynode.

3. The method of manufacturing an electron multiplier electrode comprising the steps of depositing in a vacuum a layer of material onto a substrate, said material being chosen from the group consisting of chromium, gold and nickel, depositing magnesium metal only, on said layer of material while maintaining said vacuum, oxidizing said magnesium to convert said magnesium metal to magnesium oxide, and hydrogen tiring said electrode at a temperature of about 750 C. for about 3 minutes.

References Cited by the Examiner UNITED STATES PATENTS 2,747,133 5/1956 Weimer 313-67 X 2,898,499 8/1959 Sternglass et al. 313-68 X 2,922,906 1/1960 Day et al 313-103 X 2,942,132 6/1960 Rotow et al. 313-103 3,128,406 4/1964 Joetze et al. 313-103 X 3,236,686 2/1966 Schaefer 313-103 X FOREIGN PATENTS 670,607 4/ 1952 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner,

ARTHUR GAUSS, Examiner.

S. CHATMON, JR., Assistant Examiner, 

1. IN A PHOTOSENSITIVE PICKUP TUBE COMPRISING A FLAT TARGET ADAPTED TO OPERATE AT A RELATIVELY LOW VOLTAGE WITH RESPECT TO GROUND, A HEAT RADIATING ELECTRON GUN FOR DIRECTING ELECTRONS TO ONE FACE OF SAID TARGET, A PORTION OF SAID ELECTRONS RETURNING TOWARDS SAID GUN, AN ELECTRON MULTIPLIER HAVING A FIRST DYNODE ADJACENT TO SAID ELECTRON GUN AND IN THE PATH OF SAID RETURNING ELECTRONS, SAID MULTIPLIER BEING ADAPTED TO OPERATE AT A RELATIVELY HIGH VOLTAGE WITH RESPECT TO GROUND, WHEREBY SAID RETURNING ELECTRONS DIRECTLY IMPINGE ON A SURFACE OF SAID FIRST DYNODE WITH APPRECIABLE VELOCITY AND ARE DEFLECTED TO AN ADJACENT DYNODE, THE IMPROVEMENT CHARACTERIZED IN THAT: (A) SAID FIRST DYNODE COMPRISES A COMPOSITE STRUCTURE CONSISTING OF A BASE METAL SELECTED FROM THE GROUP CONSISTING OF SILVER, INDIUM AND NICKEL, A LAYER OF CHROMIUM ON SAID BASE METAL HAVING A THICKNESS OF FROM ABOUT 100 ANGSTROM UNITS TO ABOUT 200 ANGSTROM UNITS, AND A LAYER OF MAGNESIUM OXIDE ON SAID LAYER OF CHROMIUM, SAID MAGNESIUM OXIDE LAYER BEING DIRECTLY EXPOSED TO SAID RETURNING ELECTRONS, (B) SAID MAGNESIUM OXIDE LAYER HAVING A THICKNESS RELATED TO THE VOLTAGE DIFFERENCE BETWEEN SAID TARGET AND SAID MULTIPLIER WITHIN THE RANGE OF FROM 300 VOLTS TO 600 VOLTS, SAID THICKNESS OF SAID MAGNESIUM OXIDE LAYER BEING ABOUT 75 ANGSTROM UNITS WHEN SAID VOLTAGE DIFFERENCE IS ABOUT 300 VOLTS AND INCREASING SUBSTANTIALLY LINEARLY WITH INCREASE IN VOLTAGE TO A VALUE OF ABOUT 150 ANGSTROM UNITS WHEN SAID VOLTAGE DIFFERENCE IS ABOUT 600 VOLTS, (C) WHEREBY SAID FIRST DYNODE IS CHARACTERIZED BY OPTIMUM GAIN AND IS FREE FROM ADVERSE EFFECTS FROM THE HEAT OF SAID ELECTRON GUN.
 2. THE METHOD OF MANUFACTURING A DYNODE ELECTRODE FOR USE IN AN ELECTRON TUBE HAVING A PHOTOCATHODE INCLUDING CESIUM, COMPRISING THE STEPS OF DEPOSITING IN A VACUUM A LAYER OF MATERIAL ON A SUBSTRATE BY EVAPORATION, DEPOSITIONG MAGNESIUM ONLY, ON SAID LAYER BY EVAPORATION WHILE MAINTAINING SAID VACUUM, OXIDIZING SAID MAGNESIUM, AND HYDROGEN FIRING SAID DYNODE FOR ELIMINATING CESIUM ATTRACTING CONTAMINANTS FROM SAID DYNODE. 